Abstract

The isopod crustaceans are diverse both morphologically and in described species numbers. Nearly 950 described species (∼9% of all isopods) live in continental waters, and possibly 1,400 species remain undescribed. The high frequency of cryptic species suggests that these figures are underestimates. Several major freshwater taxa have ancient biogeographic patterns dating from the division of the continents into Laurasia (Asellidae, Stenasellidae) and Gondwana (Phreatoicidea, Protojaniridae and Heterias). The suborder Asellota has the most described freshwater species, mostly in the families Asellidae and Stenasellidae. The suborder Phreatoicidea has the largest number of endemic genera. Other primary freshwater taxa have small numbers of described species, although more species are being discovered, especially in the southern hemisphere. The Oniscidea, although primarily terrestrial, has a small number of freshwater species. A diverse group of more derived isopods, the ‘Flabellifera’ sensu lato has regionally important species richness, such as in the Amazon River. These taxa are transitional between marine and freshwater realms and represent multiple colonisations of continental habitats. Most species of freshwater isopods species and many genera are narrow range endemics. This endemism ensures that human demand for fresh water will place these isopods at an increasing risk of extinction, as has already happened in a few documented cases.

Notes

Acknowledgements

Information in this article comes from communications and specimens sent by colleagues around the world. In particular, I would like to recognise important contributors of specimens and/or information from particular localities: New South Wales and elsewhere in Australia - W. Ponder; Northern Territory - C. Humphrey; Western Australia - C. Francis, S. Halse & coworkers, S. Eberhard, P. Horwitz and W. Humphreys; South Africa - G. Gouws; Brazil - C. Noro; New Zealand - D. Olsen; Chile - J. Pérez-Schultheiss. I am grateful to Marilyn Schotte (National Museum of Natural History, USA), as the maintainer of the World List of Isopods, which formed the starting point for this article. Helpful advice on the distribution of parasitic taxa was kindly given by J. Markham and J. Shields. Two referees made helpful suggestions for the revision of this article. Research on freshwater isopods at the Australian Museum has been supported by Australian Biological Resources Survey grants to myself, Stephen Keable and Chris Humphrey (Environmental Research Institute of the Supervising Scientist), and a contract from the Department of Conservation and Land Management (Western Australia). Finally, I thank Estelle Balian and Koen Martens for inviting me to the workshop and handling this manuscript.

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